A voltage controlled oscillator (VCO) is an electronic circuit for generating a periodic oscillatory output signal, e.g., an oscillatory voltage or current, the output signal having an oscillation frequency that is adjustable by means of a tuning voltage applied at a varactor in the VCO core. The oscillation frequency (also referred to as the oscillator frequency or VCO frequency) may be varied or modulated by varying this tuning voltage. VCOs may notably be used in high frequency applications such as communication and sensor systems.
One of the most important VCO signal quality parameters is the phase noise. Phase noise is a variable and unpredictable deviation of the phase of the VCO output signal and is usually undesired. Phase noise can limit the performance of an application system, and it can be one of the most critical parameters.
It has been proposed to include a current source in a VCO in order to make the VCO design robust against process and temperature variation, and also to avoid self-biasing in the VCO core. The introduction of a current source in the VCO core may, however, increase the level of VCO phase noise due to noise sources in the current source.